The inventive concept relates generally to pixels circuits and depth sensors. More particularly, the inventive concept relates to pixel circuits capable of generating a signal corresponding to a distance to an object under ambient light conditions and a depth sensor including the pixel circuit.
Generally, a complementary metal oxide semiconductor (CMOS) image sensor generates electrical charge in response to incident light received by a light receiving unit, such as a photo diode. The CMOS image sensor accumulates the charge in a floating diffusion node so that a voltage apparent at the floating diffusion node corresponds to the intensity of the incident light. Then the CMOS image sensor is able to generates an analog signal having a magnitude that corresponds to the voltage of the floating diffusion node.
Depth sensors may use a CMOS image sensor to measure (or detect) distance from a target object. In this capacity, conventional depth sensors often use a time of flight (TOF) method to approximate the distance. That is, the depth sensor may emit light towards the target object, sense light reflected from the target object, and calculate the intervening distance using the time of flight for the reflected light. Ambient light (e.g., incident environmental illumination) will affect the ability of the depth sensor to determine distance, and is typically compensated by means of an offset within TOF methods.
Those skilled in the art will recognize that when levels of ambient light are relatively high, the floating diffusion node of the depth sensor may become charge saturated as a result of ambient light. Under such circumstances, it becomes very difficult to discriminate the reflected light, and the depth sensor cannot generate an analog signal accurately indicating the relative intensity of the reflected light. As a result, the depth sensor will not provide a correct indication of distance.